From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 29842231832; Thu, 26 Feb 2026 00:29:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=216.40.44.16 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772065774; cv=none; b=qy9RqHGC/aemheeo6wuNA0mVrVHwvC8LHbwsvMgeTkcVLLy7K0cEL8otW/bYe7XLd8jOvZgFi2CWzyBV6WWPSPWy/bgq8ouhlxjpYz9WgvkqgGA9DD4S9C2gv4Ll1OK2StEW0cfOObYsA/VZYy0xoeig0tIpZ7f80upsF7yRSxc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772065774; c=relaxed/simple; bh=+7BsmtQ5iVXZanMMAoaBOnNHj3Z9QVTJ4Gaw4IIGRvE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qqAycERbjr/Nzp+wXoduWWkQUvAE0ByK+hgrcecs2L32tztIsxqQ1et5Dr2kvi7LZnpO4lj7M3AvKZaEdS8ELb/SeLQSxelnA7HcKATGuxC1MRu5OSmDoWCl3JhZCY57Fxp94L/qrRdgW4Q8TGdO0Mg+kaiw4QY8bcUE9fz533Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=groves.net; spf=pass smtp.mailfrom=groves.net; arc=none smtp.client-ip=216.40.44.16 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=groves.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=groves.net Received: from omf11.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id DBA18578AB; Thu, 26 Feb 2026 00:29:27 +0000 (UTC) Received: from [HIDDEN] (Authenticated sender: john@groves.net) by omf11.hostedemail.com (Postfix) with ESMTPA id 4843120029; Thu, 26 Feb 2026 00:29:18 +0000 (UTC) Date: Wed, 25 Feb 2026 18:29:17 -0600 From: John Groves To: Dave Jiang Cc: John Groves , Miklos Szeredi , Dan Williams , Bernd Schubert , Alison Schofield , John Groves , John Groves , Jonathan Corbet , Vishal Verma , Matthew Wilcox , Jan Kara , Alexander Viro , David Hildenbrand , Christian Brauner , "Darrick J . Wong" , Randy Dunlap , Jeff Layton , Amir Goldstein , Jonathan Cameron , Stefan Hajnoczi , Joanne Koong , Josef Bacik , Bagas Sanjaya , James Morse , Fuad Tabba , Sean Christopherson , Shivank Garg , Ackerley Tng , Gregory Price , Aravind Ramesh , Ajay Joshi , "venkataravis@micron.com" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "nvdimm@lists.linux.dev" , "linux-cxl@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" Subject: Re: [PATCH V7 19/19] famfs_fuse: Add documentation Message-ID: References: <0100019bd33b1f66-b835e86a-e8ae-443f-a474-02db88f7e6db-000000@email.amazonses.com> <20260118223420.92690-1-john@jagalactic.com> <0100019bd33ed831-615df3db-7b06-4137-9877-97c0d0fc0a05-000000@email.amazonses.com> <7facce73-688a-408a-bcf9-f16d5ff36349@intel.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7facce73-688a-408a-bcf9-f16d5ff36349@intel.com> X-Stat-Signature: e4brd93qdse8irtjpwokip6jthgpusrc X-Rspamd-Server: rspamout01 X-Rspamd-Queue-Id: 4843120029 X-Session-Marker: 6A6F686E4067726F7665732E6E6574 X-Session-ID: U2FsdGVkX1/MAbFr2gZXLy0eo5qRcyuiG2lkZq54nO0= X-HE-Tag: 1772065758-14810 X-HE-Meta: U2FsdGVkX1+x0I2cn6ZECcJvCeffIJ6JpirosHiTV51wjEmz00UjCIoh55g5Tyn6zysbWIHqXOsPhf6zCIyu1CMXr0YkF9BvHyL+v5HBbiNb8Y5oeuESDTQZh6wny1WRoCaRlIK0FcuLBDmldvL6VuHuC8/z6kClEyo60hSj0wIQahFVGTKV+oXadHuN4yr+t/ue8LSvOD1ZD5aUBTztAxZB20kMlLN/ISpZfBAN01I6FPIZOtTFmqng5mkCglBahnJ50poHId0iJkRqdoYgoc7rL9DTwrlaHI2cv99TpEbGdYXFk4OghNAOriXcMPhDY7vW1/gE5vpD25dIwLjg1qUYQ40enQF6KenGoK67kMP63USG8qnWf1mpdxHIoJBNQGkOGfjO3wACnkQdGnJLgtA+eTBeEoD1YThJQrZ4Ko8sehUgh+LYYPjYh/3XDmclx2KqUOaX1Ll/6HclL7yVRQ== On 26/02/19 02:39PM, Dave Jiang wrote: > > > On 1/18/26 3:34 PM, John Groves wrote: > > From: John Groves > > > > Add Documentation/filesystems/famfs.rst and update MAINTAINERS > > > > Reviewed-by: Randy Dunlap > > Tested-by: Randy Dunlap > > Reviewed-by: Jonathan Cameron > > Signed-off-by: John Groves > > --- > > Documentation/filesystems/famfs.rst | 142 ++++++++++++++++++++++++++++ > > Documentation/filesystems/index.rst | 1 + > > MAINTAINERS | 1 + > > 3 files changed, 144 insertions(+) > > create mode 100644 Documentation/filesystems/famfs.rst > > > > diff --git a/Documentation/filesystems/famfs.rst b/Documentation/filesystems/famfs.rst > > new file mode 100644 > > index 000000000000..bf0c0e6574bb > > --- /dev/null > > +++ b/Documentation/filesystems/famfs.rst > > @@ -0,0 +1,142 @@ > > +.. SPDX-License-Identifier: GPL-2.0 > > + > > +.. _famfs_index: > > + > > +================================================================== > > +famfs: The fabric-attached memory file system > > +================================================================== > > + > > +- Copyright (C) 2024-2026 Micron Technology, Inc. > > + > > +Introduction > > +============ > > +Compute Express Link (CXL) provides a mechanism for disaggregated or > > +fabric-attached memory (FAM). This creates opportunities for data sharing; > > +clustered apps that would otherwise have to shard or replicate data can > > s/shard/share/? Actually shard is correct here - talking about splitting data sets into shards... > > > +share one copy in disaggregated memory. > > + > > +Famfs, which is not CXL-specific in any way, provides a mechanism for > > +multiple hosts to concurrently access data in shared memory, by giving it > > +a file system interface. With famfs, any app that understands files can > > +access data sets in shared memory. Although famfs supports read and write, > > +the real point is to support mmap, which provides direct (dax) access to > > +the memory - either writable or read-only. > > + > > +Shared memory can pose complex coherency and synchronization issues, but > > +there are also simple cases. Two simple and eminently useful patterns that > > +occur frequently in data analytics and AI are: > > + > > +* Serial Sharing - Only one host or process at a time has access to a file > > +* Read-only Sharing - Multiple hosts or processes share read-only access > > + to a file > > + > > +The famfs fuse file system is part of the famfs framework; user space > > +components [1] handle metadata allocation and distribution, and provide a > > +low-level fuse server to expose files that map directly to [presumably > > +shared] memory. > > + > > +The famfs framework manages coherency of its own metadata and structures, > > +but does not attempt to manage coherency for applications. > > + > > +Famfs also provides data isolation between files. That is, even though > > +the host has access to an entire memory "device" (as a devdax device), apps > > +cannot write to memory for which the file is read-only, and mapping one > > +file provides isolation from the memory of all other files. This is pretty > > +basic, but some experimental shared memory usage patterns provide no such > > +isolation. > > + > > +Principles of Operation > > +======================= > > + > > +Famfs is a file system with one or more devdax devices as a first-class > > +backing device(s). Metadata maintenance and query operations happen > > +entirely in user space. > > + > > +The famfs low-level fuse server daemon provides file maps (fmaps) and > > +devdax device info to the fuse/famfs kernel component so that > > +read/write/mapping faults can be handled without up-calls for all active > > +files. > > + > > +The famfs user space is responsible for maintaining and distributing > > +consistent metadata. This is currently handled via an append-only > > +metadata log within the memory, but this is orthogonal to the fuse/famfs > > +kernel code. > > + > > +Once instantiated, "the same file" on each host points to the same shared > > +memory, but in-memory metadata (inodes, etc.) is ephemeral on each host > > +that has a famfs instance mounted. Use cases are free to allow or not > > +allow mutations to data on a file-by-file basis. > > + > > +When an app accesses a data object in a famfs file, there is no page cache > > +involvement. The CPU cache is loaded directly from the shared memory. In > > +some use cases, this is an enormous reduction read amplification compared > > "reduction in read amplification"? Good eye - thanks. Done. > > > +to loading an entire page into the page cache. > > + > > + > > +Famfs is Not a Conventional File System > > +--------------------------------------- > > + > > +Famfs files can be accessed by conventional means, but there are > > +limitations. The kernel component of fuse/famfs is not involved in the > > +allocation of backing memory for files at all; the famfs user space > > +creates files and responds as a low-level fuse server with fmaps and > > +devdax device info upon request. > > + > > +Famfs differs in some important ways from conventional file systems: > > + > > +* Files must be pre-allocated by the famfs framework; allocation is never > > + performed on (or after) write. > > +* Any operation that changes a file's size is considered to put the file > > + in an invalid state, disabling access to the data. It may be possible to > > + revisit this in the future. (Typically the famfs user space can restore > > + files to a valid state by replaying the famfs metadata log.) > > + > > +Famfs exists to apply the existing file system abstractions to shared > > +memory so applications and workflows can more easily adapt to an > > +environment with disaggregated shared memory. > > + > > +Memory Error Handling > > +===================== > > + > > +Possible memory errors include timeouts, poison and unexpected > > s/poison and/poison, and/ > > DJ Done, thanks! John